If the phone is placed in a proper cradle or mount, then a phone acting as a sat nav is no different than a stand alone sat nav. To make a general statement that "A phone will only be allowed to be used for its primary purpose", "If it's a phone, it's a phone." is ridiculous. The new phones are more like devices. To me, the phone feature on my iPhone is just one feature of many. I use it to access email and the internet and listen to music more than I use it as a phone. To me they are its primary functions. And now with the Sat Nav apps, it is just one more application that makes it more of a handy DEVICE, not a phone.
It's like saying that you shouldn't use a computer to play games as that is not its primary purpose, which is to process data, not for recreation. So to all those people out there playing solitaire, you better stop!
I agree that if the phone/device is not properly mounted and being held in the hand, yes fine them, but if the device is in a proper mount, then no, there should not be any fine, it is secure and out of the way and no different than any other sat nav in cars.
This is going to be touchy, and until we see the law written up and get some proper interpretation on it, not just some "spokesperson" making a comment, we will have to wait and see.
